Selective occupation of conduction band valleys in AlAs quantum wells.

We report our progress in the controlled and selective occupation of various conduction band minima in AlAs quantum wells and the corresponding magnetotransport behaviors. AlAs has three conduction band minima, or valleys, located at the X-points of the Brillouin zone. By a suitable choice of well width and in-plane stress, these minima can be occupied by two-dimensional electrons singly or in various combinations. We review the factors that determine the valley occupation and show how we have used them to produce a variety of systems with different valley occupations. In particular, we will discuss two methods that we have successfully used: (a) using a piezoelectric material to impart uniaxial strain and, (b) using a material with a different thermal expansion coefficient than our samples to impart biaxial tensile strain.